1. Field of the Invention
The present invention relates to a circuit board tester, and more particularly, to a tester for testing conduction and insulation between optional pads of a circuit board in a noncontact manner.
2. Description of the Related Art
Circuit boards increasingly have a large number of printed conductive wirings (which may be called as traces or nets, and herieinafter referred to as traces) and a large number of pads connected to the traces. This increases the time required for testing conduction and insulation among the pads. Contact testers must prepare contact probes suitable for a circuit board to be tested and control the contact probes on test pads of the circuit board. When the pads are very small, it is difficult to form an array of contact probes suitable for the pads and simultaneously and surely bring the probes in contact with the pads. It is necessary, therefore, to provide a noncontact tester to speedily test a circuit board for conduction and insulation among pads thereof. In particular, it is required to provide a tester for testing a high-density circuit board such as a multichip module (MCM) board for conduction and insulation among pads thereof.
Circuit board testing techniques are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 3-295476, 3-118484, and 4-236367.
The Publication JUPP 3-295476 (first prior art) discloses a contact tester employing a test head in which many metal contact probes are embedded. The probes simultaneously come into contact with many pads on a circuit board. A signal is applied to two arbitrary pads through the corresponding probes, and a voltage or current between the probes is measured to calculate resistance between the two pads. According to the calculated resistance, it is determined whether or not conduction between the pads is allowable if the pads are on the same trace, or whether or not insulation between the pads is allowable if the pads are on different traces.
This technique is difficult to apply to circuit boards involving very fine wiring and pads, or a great number of pads. Accordingly, a flying probe technique and a two- or four-probe technique that separately move two to four metal probes to successively measure resistance between every pair of pads, have been developed.
The Publication JUPP 3-118484 (second prior art) discloses a noncontact tester. This tester emits an electron beam to charge an arbitrary pad and a trace connected to the pad to a given voltage, to see whether or not the voltage appears on another pad or trace. If the voltage appears on another pad on the same trace, it is determined that conduction is good. If pads on another trace maintain the same voltage level before and after the charging, it is determined that insulation is good.
The JUPP 4-236367 (third prior art) discloses a noncontact tester employing a laser beam and a photoconductive sheet having a transparent conductive film. The tester emits a laser beam so that the conductive film is electrically connected to a test pad through a part of the photoconductive sheet where the laser beam irradiates, to thereby charge the test pad. Thereafter, the tester emits a laser beam to measure charges at another pad through the conductive film, thereby determining the quality of conduction and insulation between the two pads.
These prior arts have problems, however, as described below.
According to the first prior art, it is nearly impossible to fabricate a test head having an array of probes to deal with several thousands to several tens of thousands of pads of a high-density circuit board with the pads each extending several tens of micrometers square and being arranged at pitches of about 10 micrometers. It is impossible to correctly bring the probes into contact with the pads. Instead of an array of probes, four discrete probes may be employed to surely make contact with pads and apply and measure voltages. This technique, however,takes a very long time for testing. When testing 2000 pads, approximately 500 hours will be needed to carry out 2000xc3x972000 insulation tests because the probes need at least 0.5 seconds to measure resistance between each pair of the pads.
The second prior art employing electron beams may not have this kind of problem but it has another problem. The size of a circuit board is usually 10 to several tens of centimeters square, so that the tester must be three to four meters square to accommodate the circuit board in a vacuum chamber. In the vacuum chamber, the circuit board requires a long degassing time, so that it takes about one hour to start the test.
According to the third prior art, the photoconductive sheet and test pads must be completely in contact with each other. This is very difficult because the circuit board has irregularities of several micrometers.
The second and third prior arts charge an optional test pad and observe a charged state at another pad due to leakage from the charged pad. The second and third prior arts have a principle problem that it is difficult to measure correct resistance between the pads because charge and discharge time constants are affected not only by the resistance but also by the electrostatic capacitance of traces on which the pads are contained.
An object of the present invention is to provide an apparatus for carrying out conduction and insulation tests on arbitrary pads of a circuit board at high speed without employing contact probes, nor a vacuum degassing system, nor physical contact with pads. This apparatus is not affected by the electrostatic capacitance of traces of the circuit board. This apparatus reduces a delay in gate wiring and improves the operation speed of a semiconductor device.
According to the present invention, there is provided an apparatus for testing a circuit board having wiring patterns and traces with pads, comprising a unit for holding the circuit board; a unit for detecting the electrical characteristics of the circuit board, having a conductive path forming unit spaced apart from the circuit board by a predetermined gap, to form a first conductive path between a position corresponding to a first test pad on one of the traces and a first power source as well as a second conductive path between a position corresponding to a second test pad on another of the traces and a second power source; a unit for controlling a laser plasma switch in a first space between the first test pad and the first conductive path and a laser plasma switch in a second space between the second test pad and the second conductive path, the laser plasma switches being activated with laser beams to make the first and second spaces conductive; and an electrical characteristic value sampling unit included in the electrical characteristics detection unit and connected to one of the first and second conductive paths.
The conductive path forming unit may be made from laser transmission material. The conductive path forming unit may be made from photoconductive material. The conductive path forming unit may be made from photoconductive material and has a unit for supporting the photoconductive material. The photoconductive material support unit may have a function of controlling light transmission. The photoconductive material support unit may be a liquid crystal mask. The photoconductive material support unit may have a unit for controlling patterns to transmit light.
The laser plasma switch control unit may have a unit for emitting laser beams and a unit for deflecting the laser beams. The electrical characteristics detection unit may have a unit for exciting the photoconductive material.
The laser plasma switch control unit may make the first and second spaces conductive, and a predetermined time thereafter, the electrical characteristics detection unit may drive the sampling unit to detect an electrical characteristic value in the conductive paths. The electrical characteristics detected by the electrical characteristics detection unit may include resistance, voltages, currents, and insulation resistance. The apparatus may further comprise a unit such as an O-ring to seal space between a photoconductive sheet or glass plate and the circuit board, and the sealed space may be filled with a pressurized or depressurized air or rare gas such as argon or xenon.
According to the present invention, there is also provided an apparatus for testing a circuit board having wiring patterns and traces with pads, comprising a photoconductive sheet positioned above the circuit board with a predetermined gap therebetween; a light transmission pattern control unit arranged above the photoconductive sheet, for controlling the shape of light emitted from a light source, to form a first conductive path and a second conductive path on the photoconductive sheet; a first laser plasma switch control unit for controlling conduction between a first test pad of the circuit board and an end of the first conductive path; a second laser plasma switch control unit for controlling conduction between a second test pad of the circuit board and an end of the second conductive path; and a first resistance measurement unit for measuring resistance between the other end of the first conductive path and the other end of the second conductive path.
Further, according to the present invention, there is provided an apparatus for testing a circuit board having metal wiring patterns and traces with pads, comprising a photoconductive sheet positioned above the circuit board with a predetermined gap therebetween; first and second transparent comb electrodes formed on the photoconductive sheet, the teeth of the comb electrodes alternating; a first light beam control unit for emitting and controlling a light beam of predetermined shape to form a first conductive path extending from the first comb electrode to a first test pad of the circuit board; a second light beam control unit for emitting and controlling a light beam of predetermined shape to form a second conductive path extending from the second comb electrode to a second test pad of the circuit board; a unit for controlling a first laser plasma switch that controls conduction between the first test pad and an end of the first conductive path; a unit for controlling a second laser plasma switch that controls conduction between the second test pad and an end of the second conductive path; and a unit for measuring a resistance value between the other end of the first conductive path and the other end of the second conductive path through the first and second comb electrodes.
According to the present invention, there is provided an apparatus for testing a circuit board having wiring patterns and traces with pads, comprising a unit for holding the circuit board; a unit for detecting the electrical characteristics of the circuit board, having a conductive path forming unit provided with a conductive area, the conductive area being spaced apart from the circuit board by a predetermined gap and facing an area of the circuit board where all test pads are positioned, the conductive area being composed of a plurality of conductive sections that are electrically isolated from one another, one of the conductive sections being connected to a first power source, and the other conductive sections being connected to a second power source whose potential is lower than that of the first power source, the conductive sections forming conductive paths between the test pad positions and the power sources; a unit for controlling laser plasma switches that are formed in at least two spaces between the test pads and the conductive sections, the laser plasma switches being activated when irradiated with laser beams; and an electrical characteristic value sampling unit contained in the electrical characteristics detection unit and connected to one of the conductive paths.
In addition, according to the present invention, there is provided an apparatus for testing a circuit board having wiring patterns and traces with pads, comprising a unit for holding the circuit board; a unit for detecting the electrical characteristics of the circuit board, having a unit for facing one face of the circuit board with a predetermined gap between them, for forming a first conductive path between a position corresponding to a first test pad on a trace of the circuit board and a first power source, and a unit for facing the other face of the circuit board with the predetermined gap therebetween, for forming a second conductive path between a position corresponding to a second test pad on a trace, which is the same or different from the trace containing the first test pad, and a second power source whose potential is lower than that of the first power source; a unit for controlling a laser plasma switch acting in a first space between the first test pad and the first conductive path as well as a laser plasma switch acting in a second space between the second test pad and the second conductive path, the first and second laser plasma switches being excited with laser beams, to make the first and second spaces conductive; and an electrical characteristic value sampling unit contained in the electrical characteristics detection unit and connected to one of the first and second conductive paths.
According to the present invention, there is provided a method of testing a circuit board having wiring patterns and traces with pads in an apparatus, having a unit for holding the circuit board, wherein the method comprises the steps of positioning the apparatus, away from the circuit board held by the holding unit by a predetermined gap; forming a first conductive path between apposition corresponding to a first test pad on a trace of the circuit board and a first power source; forming a second conductive path between a position corresponding to a second test pad on another trace of the circuit board and a second power source; emitting a laser beam toward a first space between the first test pad and the first conductive path and another laser beam toward a second space between the second test pad and the second conductive path, to make the first and second spaces conductive; and testing the electrical characteristics of the circuit board with a sampling unit connected to one of the first and second conductive paths.
According to the present invention, there is also provided a method of testing a circuit board having wiring patterns and traces with pads, wherein the method comprises the steps of positioning a photoconductive sheet above the circuit board with a predetermined gap therebetween; employing a light transmission pattern control unit arranged above the photoconductive sheet, to control the shape of light emitted from a light source and form a first conductive path and a second conductive path on the photoconductive sheet; employing a first laser plasma switch control unit to control conduction between a first test pad of the circuit board and an end of the first conductive path; employing a second laser plasma switch control unit to control conduction between a second test pad of the circuit board and an end of the second conductive path; and employing a resistance measurement unit to measure a resistance value between the other end of the first conductive path and the other end of the second conductive path.
Further, according to the present invention, there is provided a method of testing a circuit board having metal wiring patterns and traces with pads, wherein the method comprises the steps of positioning a photoconductive sheet above the circuit board with a predetermined gap therebetween; arranging first and second transparent comb electrodes on the photoconductive sheet, the teeth of the comb electrodes alternating; employing a first light beam control unit to emit and control a light beam having a predetermined shape to form a first conductive path extending from the first comb electrode to a first test pad of the circuit board; employing a second light beam control unit to emit and control a light beam having a predetermined shape to form a second conductive path between the second comb. electrode and a second test pad of the circuit board; employing a first laser plasma switch control unit to control conduction between the first test pad and an end of the first conductive path; employing a second laser plasma switch control unit to control conductive between the second test pad and an end of the second conductive path; and employing a resistance measurement unit to measure a resistance value between the other end of the first conductive path involving the first comb electrode and the other end of the second conductive path involving the second comb electrode.
According to the present invention, there is provided a method of testing a circuit board having wiring patterns and traces with pads in an apparatus. having a unit for holding the circuit board, wherein the method comprises the steps of positioning a conductive area facing the circuit board with a predetermined gap therebetween, to cover an area of the circuit board where all test pads are disposed; dividing the conductive area into a plurality of conductive sections that are electrically isolated from one another; connecting at least one of the conductive sections to a first power source; connecting another of the conductive sections to a second power source whose potential is lower than that of the first power source; forming conductive paths between test pad positions and the power sources; employing a laser plasma switch control unit to emit laser beams toward spaces between at least two test pads and the conductive sections and make the spaces conductive; and employing an electrical characteristics detection unit to detect the electrical characteristics of the conductive paths.
Further, according to the present invention, there is also provided a method of testing a circuit board having wiring patterns and traces with pads in an apparatus, having a unit for holding the circuit board, wherein the method comprises the steps of arranging a first conductive section facing the circuit board held by the holding unit with a predetermined gap therebetween, to secure conduction between a position of a first test pad on a trace of the circuit board and a first power source; arranging a second conductive section facing the circuit board held by the holding unit with a predetermined gap therebetween, to secure conduction between a position of a second test pad on another trace of the circuit board and a second power source, the second conductive section being isolated from the first conductive section; employing a laser plasma switch control unit to emit a laser beam toward a first space between the first test pad and the first conductive section and another laser beam toward a second space between the second test pad and the second conductive section, to make the first and second spaces conductive; and employing an electrical characteristic value sampling unit connected to at least one of the first and second conductive sections, to test the electrical characteristics of the circuit board.
In addition, according to the present invention, there is provided a method of testing a circuit board having wiring patterns and traces with pads in an apparatus having a unit for holding the circuit board, wherein the method comprises the steps of forming a first conductive path between a position corresponding to a first test pad on a trace on one face of the circuit board and a first power source; forming a second conductive path between a position corresponding to a second test pad on a trace, which is the same or different from the trace containing the first test pad, on another face of the circuit board and a second power source whose potential is lower than that of the first power source; employing a laser plasma switch control unit to emit a laser beam to a first space between the first test pad and the first conductive path and another laser beam toward a second space between the second test pad and the second conductive path, to make the first and second spaces conductive; and employing an electrical characteristic value sampling unit connected to one of the first and second conductive paths, to measure electrical characteristic values of the circuit board.